1. Field of the Invention
The present invention relates to compositions and processes for coating metals and, more particularly, to aqueous compositions of metals for producing conversion surfaces without the use of electromotive force, processes for making these aqueous compositions, and products produced by these processes which provide improved friction-reducing modifiers.
2. Technical Background
It is known that thin mono-molecular oxide films present on stainless steel can provide an excellent passivation surface to metals. It has been theorized that corrosion may one day be conquered by a thin molecular layer on metal surfaces. It has been further theorized that significant reductions in friction could be obtained with thin, tenacious metallic films.
Many methods have been developed to form conversion surfaces on metals, including electroplating, phosphating in chemical vapor deposition, and ion sputtering. Conversion surfaces may be very effective in reducing friction, compared to conventional oil lubricants, but they are expensive to create and require complex methods. Lubricants create a boundary layer between two surfaces which keeps the two surfaces apart. When the lubricant can no longer maintain the boundary the surfaces come into contact and wearing of the surfaces occurs due to friction. Lubricants form only transitory boundary layers and have limited use in reducing friction. Conversion coatings, on the other hand, create relatively long-lasting boundary layers and are more effective in reducing friction. A conversion surface consisting mainly of metal is expected to most likely approach a frictionless surface. Defalco and McCoy (U.S. Pat. No. 5,540,788) demonstrated that molybdenum, zinc, or tungsten can be deposited as a conversion coating on an iron surface when the salts of these metals are first dissolved in an inorganic phosphate polymeric water complex and then delivered in an oil lubricant vehicle to the iron surface. The polymeric water complex by itself forms a phosphate and potassium conversion surface on an iron surface when delivered in the lubricant vehicle. The phosphate/potassium conversion service by itself significantly improved the friction reducing properties of the lubricant vehicle. Adding molybdenum, zinc, or tungsten to the polymeric water complex did not produce an improved anti-friction effect compared to the polymeric water complex alone. In addition the reactions disclosed by Defalco and McCoy require a violent chemical exothermic reaction which makes this process unfeasible commercially. Defalco (US Patent Application No. 2008/0302267) further demonstrated that metal ions and silicate can form conversion coatings on any metal surface if the aqueous delivery media of inorganic phosphate solution was formulated not to form polymeric complexes. His formulation required no violent chemical exothermic reaction. A dry conversion coating of silicon on stainless steel provided the same anti-friction protection by itself as a lubricating oil, using this formulation.
In the October 1996 issue of Scientific American, Jacqueline Krim, PhD, published a paper titled “Friction at the Atomic Scale”. Her findings led to the conclusion that “at the atomic level with metal to metal contact there is no friction.” This surprising finding called into question many of the beliefs that friction was a condition that could only be alleviated by the use of a lubricant to reduce the heat generated by metals sliding over one another. Another surprising conclusion was that, at the atomic level, “friction arises from atomic lattice vibrations when atoms close to the surface are set into motion by the sliding action of atoms in the opposing surface. These vibrations are really sound waves. In this way, some of the mechanical energy needed to slide one surface over the other, is converted to sound energy, which is eventually transformed into heat.” Heat causes friction. To maintain the sliding, more mechanical energy must be added. Krim further posits “Solids vibrate only at certain distinct frequencies, so the amount of mechanical energy depends on the frequencies actually excited. If the atoms in the opposing surface resonates with the frequency of the other surface, then friction arises. But if the opposing surface is not resonant with any of the other surface's own frequencies, then sound waves are not generated. This feature opens the exciting possibility that sufficiently small solids, which have relatively few resonant frequencies, might exhibit nearly frictionless sliding.”
Another surprising result of her work was that dry films were slipperier than liquid films. This was counterintuitive to all current thought on friction. Further tests by other scientists validated that metal to metal contact at the atomic level eliminated friction, and that liquid lubricants caused friction with the “stick/slip” action. The liquid would stick in the gaps in the metal and then slip out. This caused vibrations in the lattices and generated sound waves which converted to heat, causing friction.
Although inorganic aqueous solutions containing salts of non-alkaline metals can be formulated to create non-alkaline metal conversion coatings on metals without the use of external electromotive force, they do not appear to offer an advantage over standard liquid or dry organic lubricating agents for reducing friction.